There are numerous types of respirator helmets. Many respirator helmets have an interior space between the head and the interior surface of the respirator helmet. Within that space, the respirator helmet is designed to forward a sufficient amount of air toward the nose and mouth of the user of the respirator helmet. The air is forwarded toward the user's breathing orifices by either a built-in air filter and fan, or by a remote air supply system that feeds the air into the helmet interior through a suitable tube or pipe. Containing the air about the user's orifices is accomplished through a suitable design of a full-face visor, normally transparent. Examples of these respiratory helmets are disclosed in U.S. Pat. Nos. 4,590,951, 4,097,929 and 4,136,688, all of which are commonly assigned to Racal Limited or subsidiaries thereof.
The invention disclosed in the '929 patent is a protective visor. The visor “comprises an arcuately curved frame having an aperture arranged to accept an arcuately flexed rectangular sheet of resilient transparent material. The sheet is retained in the frame by lugs extending into the aperture at staggered positions on the inner and outer margins of the upper and lower frame bars and has at the sides of the aperture recesses in the frame side bars into which the lateral edges of the transparent sheet will snap. The sheet may be of transparent polycarbonate [material] and the frame of either transparent or opaque polycarbonate. Preferably the frame has at its upper corners hinge members for attachment to a protective helmet.”
The helmet disclosed in the '929 patent, however, was apparently not sufficiently dust proof because Racal filed another application that matured into the '688 patent that addressed that problem. In particular, Racal suggested using bristles within the helmet to solve this problem.
In each helmet illustrated in the '929 and '688 patents, there is a hard helmet having an exterior and interior surface, a visor that rotates about a single point immediately above the user's ears, an air space between a user's head and the interior surface, and an aperture on the rear of the helmet to receive an air tube. This air space is where the air from the air tube traverses through the helmet. The air is pushed through this air space by a fan, which is positioned near the aperture, within the interior surface, and spaced away from the user's head by a second interior wall. The air enters the interior of the helmet, and is pushed into the area between the visor and the user's mouth.
After a number of years, Racal filed another application that matured into the '951 patent. The '951 patent illustrates a different embodiment of a respiratory helmet. Instead of the air hose entering the helmet at or near the anterior neck, the air tube enters the helmet near the user's mouth. As such, Racal was working on alternative embodiments of a respirator helmet to correct the problems of the previous models, some which are mobility of the head and dirt permeating through the shield due to a poor pivot point.